1. Field of the Invention
This invention relates to a thermal recording element for recording information by thermally deforming the recording layer thereof by exposure to high intensity radiation and more particularly it relates to a thermal recording element having protective layers formed for increasing the sensitivity of the recording layer.
2. Description of the Prior Art
As recording elements for recording information by exposing to high intensity radiation such as a laser beam, etc., there is the following thermal recording element in addition to light-sensitive elements using silver salts, etc. That is, in the thermal recording element, the recording layer has a high optical density and produces a local temperature increase by absorbing high intensity irradiation, whereby the irradiated portions are removed by thermal deformation such as fusion, evaporation, aggregation, etc., to record information as the difference in optical density between the irradiated portions and non-irradiated portions. Such a thermal recording element has the advantage that processing such as developing, fixing, etc., are unnecessary; a dark room is unnecessary for recording since the element is not sensitive to ordinary room light, high contrast images are obtained, and additional recording (add on) of information is possible.
Recording on such a thermal recording element is generally performed by converting a recording information to an electric time-succession signal and scanning the recording element with a laser beam the intensity of which is modulated according to the electric signal. In this case, there is the advantage that the recorded image is obtained in real time.
As the recording layer for such a thermal recording element, metals, dyes, plastics, etc., are suitably used and, in general, an inexpensive material can be used. Such recording elements are described in, for example, M. L. Levene, Electron, Ion and Laser Beam Technology (records of the 11th Symposium held in 1969); Electronics, p. 50, Mar. 18, 1968; D. Maydan, The Bell System Technical Journal, Vol. 50, 1761 (1971); C. O. Carlson, Science, Vol. 154, 1550 (1966), etc. Some of these recording elements have metal recording layers. For example, such a recording element comprises a support having formed thereon a thin layer of a metal such as Bi, Sn, In, etc., and has excellent properties as a thermal recording element since an image of high resolving power and high contrast can be recorded thereon.
However, the recording element containing a metal thin layer shows generally light reflectance of higher than 50% to laser light used for recording, which makes it impossible to effectively utilize the energy of the laser light. Hence the power of the laser beam used for recording must be higher, which requires a laser source of high output capable of recording at high scanning speeds, and thus the recording apparatus used becomes larger and expensive.
Therefore, various recording elements having high recording sensitivity have been studied and a recording element of three layer structure composed of Se, Bi and Ge is disclosed in Japanese Patent Publication No. 40479/71. In this element to reduce the light reflectance of a thin layer composed of Se and Bi, a thin layer of Ge is formed on the layer of Se and Bi. However, the use of Se, etc., is undesirable since there is a possibility of a toxicity problem and the images recorded are not satisfactory.
As another example of a recording element having a reflection preventing layer, a recording element having on a metal layer a reflection preventing layer having light absorption in the wavelength region of the laser beam used for recording is disclosed in Japanese Patent Application (OPI) No. 74632/76 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application"). However, even with a reflection preventing layer, it is very difficult to completely eliminate the occurrence of light reflection and if the occurrence of light reflection can be completely eliminated, a laser light source of high power is required to accomplish thermal deformation such as fusion, evaporation, aggregation, etc., of the metal thin layer and a recording element having higher sensitivity is desired.
Since the recording layer, e.g., a thin metal layer of the thermal recording element as mentioned above is liable to scratch, a protective layer is formed on the metal layer of the recording element to improve the durability, mechanical strength, time stability, etc., of recording element. The protective layer must transmit the light beam of high energy density used for recording, it must have high mechanical strength, it must be slow to react with the recording layer, exhibit good coating property, etc.
As materials for the protective layer, inorganic materials and organic materials have been used. As the inorganic materials, there are transparent materials such as Al.sub.2 O.sub.3, SiO.sub.2, SiO, MgO, ZnO, MgF.sub.2, CuF.sub.2, etc. The use of inorganic materials as the protective layers is an excellent practice and is disclosed in, for example, Japanese Patent Application (OPI) Nos. 96716/74, 59626/76, 75523/76, 88024/76 and 134633/76. However, the recording elements having protective layers described in the above specifications suffer the disadvantages that the recording sensitivity is greatly reduced as compared with that of a recording element having no protective layer.
When a protective layer composed of an organic high molecular weight material is used, in order to impart sufficient mechanical strength to the recording element the thickness of the protective layer generally must be at least 3 .mu.m and, in this case, a recording energy of 2 to 3 times that in the case of no protective layer is required. If the thickness of the protective layer is thinner than above, in particular, is thinner than 1 .mu.m, the reduction in sensitivity becomes less but the mechanical strength of the recording element is reduced too much to make the recording element suitable for practical use.
Thus, in the case of using the protective layer composed of an organic high molecular weight material or a polymer, it is most important to limit the reduction in recording sensitivity as much as possible while maintaining the strength of the protective layer at a level adequate for practical use.
It can generally be said that when the protective layer having good adhesive property to a recording layer, good film-forming property, a high softening point and a thick thickness is used, the reduction in sensitivity is great but the mechanical strength of the protective layer becomes sufficiently high, while, on the other hand, when the protective layer composed of a polymer having poor adhesive property to a recording layer, poor film-forming property, a low softening point, and a thin thickness is used, the reduction in sensitivity is comparatively less but the mechanical strength of the protective layer is insufficient. As is clear from the above description, the formation of a protective layer capable of protecting the recording layer and giving less reduction in recording sensivity lies in balancing conflicting properties, for example, the poor film-forming property and the high mechanical strength of the layer.